Conventional photoconductive detectors comprise one or more square elements of photosensitive material, each element having a pair of spaced bias contacts. For imaging applications, such a detector is placed in the image plane of an optical assembly and is usually shielded to reduce the incidence of background illumination upon the detector. The detector is usually mounted on a cold stage and is cooled to enhance signal-over-noise discrimination. In one form of conventional detector apparatus using intrinsic photoconductive elements responsive to the middle and far-infra-red region of the spectrum, a steady direct current (DC) bias, from a constant current source, is applied to each element. There is thus developed across each detector element a bias pedestal voltage, a voltage dependent on bias current magnitude and element resistance. When radiation of appropriate wavelength is incident upon the detector elements, photoresponse signals--in this case photovoltages--are developed and these increment the voltage provided by each element. The incremental photoresponse signal voltage is, for normal radiation intensities, of magnitude several orders smaller than the magnitude of the bias pedestal, and it is usual to back-off each element voltage by subtracting DC voltage to allow extraction and amplification of the photosignal. However, to be wholly effective the back-off voltage applied, in each case, must follow changes in the pedestal voltage. Such changes may occur, for example, as a result of cold stage temperature drift, of change in ambient temperature, of change of average background illumination, and of bias current drift. Such pedestal voltage changes are in general also orders of magnitude higher than the photosignal increment. Furthermore the pedestal voltage and the change of this voltage will vary from element to element. In general the resistance of each element will differ, since material resistivity and element dimensions vary within manufacturing tolerance. Because of non-uniformity in the bias pedestal, it is in the very least difficult, if not impractical, in unscanned, so called "staring" systems, to back off element voltage satisfactorily so that the wanted illumination dependent photoresponse signal can be extracted without the introduction of an unacceptable degree of fixed pattern noise. It is also possible to operate these detectors using constant voltage drive bias instead of constant current in which case device current is measured. This too requires bias compensation, and this likewise introduces fixed pattern noise.
Because of these difficulties, progress in photoconductive detector development is impeded and this development is giving way to the alternative development of photovoltaic detectors, albeit this latter involves a more complex, generally more expensive and less far advanced technology.
Alternative to the use of DC bias, experimental use of microwave frequency alternating current bias has been reported in the literature (A. S. Sommers Jr, Microwave biassed photoconductive detector, Chapter 11 (page 435) Semiconductors and Semimetals Vol 5, Eds Willardson and Deer (Academic Press) 1970). The DC responsivity (ie voltage increment for unit intensity of radiation of appropriate wavelength) of a photoconductive element is limited by photocarrier recombination losses occurring at the bias contacts. However, using very high frequency alternating bias as reported, it is possible to reduce these forced recombination losses, since the flow direction of the photocarriers can be reversed before many of the photocarriers reach the bias contacts. In this case the photocarrier density is limited instead by natural recombination losses in the element material bulk, these carriers recombining within a natural average lifetime. Much higher linear responsivity (AC) is claimed to be attained. However, as reported, the detector is biassed in the microwave field of a tuned microwave resonant cavity. Such apparatus is complex, difficult to set up accurately, and is expensive. So far as is known, such apparatus has not been applied commercially. The principle merit is that it allows the photoconductor to be biassed at high fields without suffering the consequential effects of carrier loss at the contacts. The developed photoresponse signal and bias response however are both linear and suffer the same fixed pattern noise problem.